The present invention relates to the formation of an oxide film on the surface of a valve-metal and more particularly to a continuous process of formation of an oxide film by electrolytic means on an etched valve-metal foil. Such formed foil is typically used in the manufacture of electrolytic capacitors. The word "forming" and variations thereof are used herein to mean anodizing, as is common practice in the art.
Formed foil is conventionally made by drawing a bare metal foil strip through a liquid electrolyte solution and applying a d.c. voltage between the foil and a negative electrode that is also submersed in the electrolyte solution. The foil to be formed has normally been etched so as to increase the effective surface area. The ratio of the area after etching to the original plane area is called the etch ratio.
Etched foil that is typically provided for such purposes does not have a uniform etch ratio. This lack of etch ratio uniformity causes the formation current to vary in a continuous formation process. The variations in formation current result in changes in the voltage drops in the electrolyte and the associated circuitry. The ultimate result is undesirable variations in the oxide film thickness. Furthermore, the d.c. power supply employed in the continuous formation process cannot be operated at its maximum current rating since current variations must be allowed for in establishing the normal or average current drawn from it. This means that for a given formation facility, operation at maximum loading is never possible. Since the d.c. power source, for example a d.c. generator, represents a major portion of the investment in a formation facility, this is an important consideration, adversely impacting the cost of manufacturing formed foil.
It is therefore an object of the present invention to provide a continuous method for forming a uniformly thick oxide film on a valve-metal foil.
It is a further object of the present invention to provide an economic continuous method for forming an oxide film on a valve-metal foil.
It is yet a further object of the present invention to provide a continuous method for forming an oxide film on a valve metal wherein the d.c. power source may be operated continuously at its maximum rated output.